As an apparatus-to-apparatus or an apparatus-constituent unit-to-unit data communication method, a traditional RS232C interface, which is a simple and start-stop synchronous type serial communication means, has widely being used.
However, as to a unit-to-unit data communication in an apparatus, a proper counter plan must be considered against events requiring real timing with less time skew (real-time events). Real time events mean, for example, in data communication between a main control unit and an input/output unit under the control of the main control unit, any trouble in the input/output unit, any change in a sensor contained in the input/output unit, driving of a motor contained in the input/output unit, etc. And, in the case when the main control unit directly controls the input/output unit, a state control signal corresponding to these real time events must be communicated between the unit-to-unit, without time skew. For this purpose, for instance, the main control unit and the input/output unit will be directly connected for every necessary signal (that is to say, a parallel connection of the number of signal lines).
FIG. 12 illustrates communication of the state control signal. In FIG. 12, a CPU 11 of a main control unit 10, a motor 24 and a sensor 25 of an input/output unit 20 are directly connected in parallel with the required number of signal lines (in the drawing, 4 signal lines, A, B, C and D). And, for example, a motor drive signal from the CPU 11 of the main control unit 10 is directly supplied to the motor 24 of the input/output unit 20, through the private signal lines A, B and C. Also, a detection signal from the sensor 25 of the input/output unit 20 is directly supplied to the main control unit 10, through the private signal line D. By these systems, the state control signal can be transferred without any time skew. Moreover, the motor drive signal or the detection signal is one of the state control signals.
In addition, the input/output unit 20 itself can control itself, mounting a MPU, which is a control means. However, even in this case, as the input/output unit controls itself based on a control command from the main control unit 10, a specified control command signal must be communicated between the CPU 11 of the main control unit 10 and the MPU of the input/output unit.
FIG. 13 illustrates communication of the control command signal. In FIG. 13, a parallel control command signal from the CPU 11 of the main control unit 10 is once stored in a control command sender register 121-1 connected to the CPU 11 with a bus. After that, the control command signal is converted into a serial signal by a P/S conversion data sender section 121-3, and sent to the input/output unit 20. The input/output unit 20 converts the serial signal into a parallel control command signal by a S/P convert data receiver section 122-3, and stores the converted data in a control command receiver register 122-1. After that, the control command signal is supplied to a MPU 21. The MPU 21 directly connects to a motor 24 and a sensor 25 in the unit. Also, the control command signal from the MPU 21 of the input/output unit 20 will be sent through the control command sender register (not noted in the drawing) and the P/S convert data sender section (not noted in the drawing) contained in the input/output unit 20, and supplied to the CPU 11 of the main control unit 10, through the S/P convert data receiver data section (not noted in the drawing) and a control command receiver register (not noted in the drawing). Thus, the control command signal is transferred unit-to-unit by serial communication.